In designing of the wind turbine generator, optimization of the connection structure between a main shaft and a generator rotor is one of important design particulars. The main shaft and the generator rotor have to be connected with a necessary strength. Meanwhile, it is desired for the main shaft and the generator rotor to be connected each other so as to allow relative movement in case of an excessive torque application between the main shaft and the generator rotor, for example, in case of a short-circuit in the generator.
One of commonly-known structures as a connection structure between the main shaft and the generator rotor is a scheme in which the rotor plate of the generator rotor is connected to a cylindrical sleeve and the cylindrical sleeve is tightening by a shrink-fit (a shrink disk). The structure is preferable in that the connection strength between the main shaft and the generator rotor can be easily adjusted and additionally the cost can be reduced, since the generator rotor can be connected to the main shaft by a generally-available shrink-fit.
The structure for connecting the main shaft and the generator rotor by using a sleeve and a shrink fit is described, for example, in International Publication Pamphlet WO2007/111425 A1 (Patent Document 1). Patent Document 1 discloses a connection structure for tightening a rotor bearing (corresponding to a sleeve of the present invention) to which the generator rotor is connected to the main shaft by a shrink disk (corresponding to a shrink fit of the present invention). In the connection structure, the shrink disk is placed between the power generator and a bearing, and tightens the rotor bearing to the main shaft at that position. The shrink disk is composed of a fixed disk and a movable disk, and the fixed disk and the movable disk are connected with each other by a bolt. The bolt is inserted in parallel with the central axis of the main shaft. When the bolt is tightened, the movable disk is pressed to the fixed disk to narrow the internal diameter of the shrink disk and accordingly the rotor bearing is tightened to the main shaft.
One problem of such connection structure is an increase of the clearance between the bearing supporting the main shaft and the power generator. As described above, the connection structure of Patent Document 1 requires securing a work space used for tightening the bolt between the power generator and the bearing, since the shrink disk is positioned between the power generator and the bearing and the bolt is inserted in a direction parallel to the main shaft. This increases the clearance between the power generator and the bearing. The increase in the clearance between the power generator and the bearing increases bowing of the main shaft, enhancing vibrations. This is not preferable because the mechanical load is increased. The increase of the clearance between the power generator and the bearing is a problem in that the length of the main shaft is increased. The increase in the length of the main shaft also increases the cost of the main shaft.
The problem of the increase in the clearance between the power generator and the bearing is serious especially in a direct-drive wind turbine generator. In the direct-drive wind turbine generator, which does not use a speed-up gear, the stator pole number and the rotor pole number are required to increase, and accordingly the weight of the power generator tends to increase. In this case, when the distance between the bearing and the power generator is increased, this may further increase the vibration, due to the further increase in the bowing of the main shaft.